1. Field of the Invention
The present invention relates to an optical head device for recording and reading information on a multi-layered optical recording medium having a plurality of recording layers, and to an optical recording and reading system which includes the optical head device.
2. Description of the Related Art
In Japanese Patent Application Laid-Open No. Hei 8-185640, there is disclosed an optical pickup device for use with 2-layer optical discs which accurately detects a servo signal and performs control operations without fail.
The optical pickup device includes: a photodetector for receiving a reflected beam from a two-layer optical disc; a first focusing lens disposed between the photodetector and a beam splitter with an optical axis of the first focusing lens aligned therewith; a light shield plate for shielding a reflected beam from an unfocused information signal layer; and a second focusing lens. The optical pickup device is configured to prevent the reflected beam from the unfocused information signal layer, i.e., inter-layer stray light from reaching the photodetector in order to detect the servo signal with accuracy.
However, with the optical pickup device disclosed in Japanese Patent Application Laid-Open No. Hei 8-185640 mentioned above, it was difficult to adjust the position of the light shield plate, i.e., a pin hole plate for shielding a reflected beam from an unfocused information signal layer. Additionally, a large distance between the two focusing lenses leads to an increase in the size of the optical pickup device. The use of focusing lenses also disadvantageously increases the number of parts required.
The inventor has found, as a result of intensive studies, that shielding with a shield plate at the front focal line position of an astigmatic optical element used greatly reduces adverse effects the inter-layer stray light had, even by the shielding at a position substantially away outwardly from the center of the reflected light beam.